The original Raspberry Pi SoC (BCM2835) is ARMv6 with VFP2 Hard Float support.
Debian's "arm" architecture is ARMv7 with VFP3. It doesn't support BCM2835.
Debian's "armel" architecture is ARMv4. It doesn't use BCM2835 to its full potential.
So the BCM2835 is awkwardly positioned in-between Debian's two stock ARM 32-bit architectures, which motivated the decision to recompile all packages for a BCM2835-specific "armhf" distribution.
Debian's "arm" architecture is ARMv7 with VFP3. It doesn't support BCM2835.
Debian's "armel" architecture is ARMv4. It doesn't use BCM2835 to its full potential.
So the BCM2835 is awkwardly positioned in-between Debian's two stock ARM 32-bit architectures, which motivated the decision to recompile all packages for a BCM2835-specific "armhf" distribution.
In a sense, it's a historic artifact.